1. Technical Field
The present invention relates generally to a multi-band filter used in a multi-channel communication system, such as a two-way CATV cable system. The present invention relates more particularly to a multi-band filter providing a predetermined amount of attenuation in one of the bands of said filter.
2. Background Art
In some multi-channel communication systems, it is desirable to selectively control the loss at several points in at least one of the channels of the system. One example of such a system is a community antenna television (CATV) system, having a two-way communication capability. In such a system, RF television program signals are transmitted from a headend to a subscriber end over a particular frequency band. This band is referred to as the forward (or downstream) path (or channel) of the CATV system. Other signals are transmitted from the subscriber end to the headend, or to some other upstream station. These other signals are transmitted in a different frequency band (higher and/or lower) than the forward path frequency band, and this different band is usually referred to as the return (or upstream) path (or channel) of the CATV system.
Coupled between the headend and subscriber end of the CATV system, there is a system of coaxial and/or optical cables. A typical cable system architecture includes a main trunk cable connected between the headend and a trunk/bridger station. One or more feeder cables feed off of the trunk/bridger station. Each feeder cable contains a number of taps disposed along the length of the feeder cable, and each tap contains a number of ports. A drop cable is connected between each port and a subscriber end. A television receiver is located at the subscriber end, and for a system offering two-way communications, the subscriber end will also have a terminal which transmits signals upstream, in the return path of the cable system.
Cable operating companies are now offering to their subscribers advanced communication services in the return path, including addressable converter operation, Pay Per View transactions, telephony, interactive digital networks, and computer data transmission. In order to offer such services in a reliable manner, certain problems in the return path must be addressed. For instance, many CATV cable systems are designed primarily for forward path operation. The loss (or attenuation) values of each tap are selected to provide proper signal levels at the drop cables, at forward path frequencies. The forward signal at each successive tap port is designed to have the same level at the highest design frequency. This insures a proper forward signal level to each subscriber.
Due to the forward tap design, the loss in the return path varies widely with every tap. This causes a corresponding variance in the signal levels in the return path. This variance in signal level imposes severe design constraints on subscriber terminal transmitters (e.g., set-top addressable converters), and adversely affects the ability of headend receivers to properly detect the return path signals. Significant improvements in the return path performance can be achieved by controlling the loss variance in the return path. If the loss at each tap port can be made substantially uniform, the total variance can be brought down to an acceptable level.
Another problem that must be addressed is interference (or "ingress") entering the return path at the subscriber end. Such interference is caused by radio frequency, electrical and electromagnetic sources, and enters the return path through damaged or insufficiently shielded subscriber drop cable, corroded "F" connectors, and internal subscriber wiring and hardware. As a result of the forward tap design, the taps contributing most to ingress are those having lower attenuation values (i.e., those taps at the distal end of the feeder cables).
An article by Dean A. Stoneback and William F. Beck, entitled "Designing the Return System for Full Digital Services", dated 1995, p. 269-71, (hereinafter "Stoneback" Article"), suggests that by reducing the loss variance in the return path (i.e., equalizing or balancing the return path), ingress can be reduced to a manageable level. Thus, the Stoneback Article suggests that by addressing the loss variance problem, the ingress problem can be satisfactorily resolved in most cases.
A practical and effective approach to balancing the return path loss must satisfy several requirements. First, the devices used to balance the loss must be inexpensive because of the large number of such devices needed to be distributed throughout the CATV cable system. Second, the devices must be small and light weight, because such devices need to be deployed on cables, tap ports, within the taps themselves, inside network interface devices, in on-premise enclosures, pedestals, ground blocks, etc. In many applications, the device should be housed in a small housing containing connectors at each end. Third, it is most desirable to leave unaffected the forward path response when employing the loss balancing devices. The loss chosen for the return path should be independent of the forward path response.
The Stoneback Article suggests two alternative methods of balancing the return path loss. The first is to employ a diplex filter with flat loss in the return band. The second method is to use an equalizer which covers the entire forward and return frequency bands. The diplex filter approach is discouraged by the Stoneback Article, because of the heretofore prevailing view that the device which adds flat loss in the return path is more difficult to manufacture.
Diplex filters have been used for processing both the forward and return paths in a CATV cable system. Examples of such use are disclosed in U.S. Pat. Nos. 5,434,610 to Loveless; 5,425,027 to Baran; 5,130,664 to Pavlic et al.; and 4,963,966 to Harney et al. The patents to Loveless and Pavlic show conventional circuit architectures for diplex filters employing flat loss. Such architectures employ a network of separately connected devices, each of which has a separate housing and associated connectors. Such assembled networks are more susceptible to interference pick-up than a singly packaged network. In addition, such architectures are too large and expensive for the CATV loss balancing application hereinabove described. Furthermore, as mentioned by the Stoneback Article, the flat loss device is more difficult to manufacture than an equalizer approach.
The use of equalization filters, as described in the Stoneback Article, also has drawbacks in the contemplated CATV loss balancing application. The forward path system architecture must be reconfigured, usually with an unfavorable reduction in system dynamic range performance. In addition, the loss chosen for the return path is not independent of the loss established in the forward path. Moreover, wideband (e.g., 5-750 MHz) equalization filters are more difficult to design and match than diplex filters. An example of a CATV system employing an equalizer is disclosed in U.S. Pat. No. 5,379,141 to Thompson et al.